A young girl plays with her mother near pedestrians and passers-by at the City of London boundary griffin on London Bridge. The City of London is a geographically small City within Greater London, England, the historic core of London from which, along with Westminster, the modern conurbation grew. The City's boundaries have remained constant since the Middle Ages but it is now only a tiny part of Greater London. This is a major financial centre, often referred to as just the City or as the Square Mile, as it is approximately one square mile (2.6 km) in area. London Bridge's history stretches back to the first crossing over Roman Londinium, close to this site and subsequent wooden and stone bridges have helped modern London become a financial success.